1. Field
The present disclosure relates generally to gallium nitride (GaN) substrates, and, more specifically, the present disclosure relates to a method for fabricating a GaN substrate on silicon.
2. Description of Related Art
Wide-bandgap semiconductors are widely used for fabricating active devices for high-voltage applications. One type of semiconductor structure known as a heterostructure field effect transistors (HFET) (also called high-electron mobility transistors (HEMT)) uses wide-bandgap semiconductors to implement transistors for high-performance power electronics. In one example, wide-bandgap HFET devices may be used as a switching element in high-voltage switching power converters.
GaN is one example of a wide-bandgap semiconductor that has generated particular interest. For example, AlGaN/GaN HFETs show promise for power electronics due to their wider bandgap and high-electron saturation velocity, both of which enable high-voltage operation. However, the difficulty and expense in forming GaN substrates have limited the application of GaN-based devices to specific markets.
A GaN substrate is typically manufactured by growing GaN film on another substrate, due to the expense and difficulty of manufacturing bulk GaN wafers. For example, silicon carbide (SiC) or sapphire (Al2O3) wafers may be used as handle wafers for a GaN substrate (i.e., a GaN film is deposited over the handle wafer). However, sapphire is a poor thermal conductor and can present difficulties during packaging, and SiC wafers are still very expensive. Moreover, both types of wafers are only available as smaller-diameter wafers, which eliminate the economy of scales available with larger diameters.
Another option for creating GaN substrates is using a silicon handle wafer, which is inexpensive and available in large diameters. Additionally, backend grinding and lapping needed for packaging is well developed for silicon wafers. However, due to a large lattice mismatch and large thermal mismatch between GaN and silicon, it may be difficult to reliably grow GaN directly on a silicon (Si) substrate. Instead, the epitaxial growth of crack-free GaN on silicon may require extensive buffer layer engineering to minimize the bow and warp during and after growth. In addition, high-voltage applications (e.g., above 600 V) may require buffer layers in excess of 2.5 μm and even up to 4 μm (e.g., for 1,000 V applications).